1. Field of the Invention
The present invention relates to data communication transceivers using base band packet switching, sometimes known as the Ethernet system.
2. Description of the Prior Art
In prior art data communications transceivers, the receiver part of the transceiver is either direct coupled or capacity coupled to the communicating medium, such as a shielded coaxial cable. In the former case, the voltage level of the signal will affect the skew of the received data. In the latter case, noise or reflection on the transmission line can be coupled to the receiver which in turn will be received as an unwanted signal. Furthermore, the capacitive, that is, AC coupling, will limit the low frequency data to the extent of the differentiation being used by the coupling.
Another problem is that when the transceiver is transmitting, electromagnetic interference may be present around the connection between the connector to the shielded cable and the shielded container of the transceiver. This electromagnetic interference is believed to be due to stray capacitance between the center conductor of the connector and the shielded container. A conventional way to minimize this effect is to shield the exposed center conductor or to make the outer connector much more capacitive relative to the shielded container than that of the center conductor. This can produce switching spikes which cause unwanted interference in the atmosphere around the transceiver because of high speed switching and this AC coupling to ground.
Still another problem occurs where the transmission medium, that is, the coaxial cable, is very long. In such cases there is a tendency for the waveform of the transmitted pulses to be flattened and extended on top of a DC level which increases in amplitude with the length of the transmission line. In order to receive and reconstruct these pulses, a Schmitt trigger is sometimes used, however, such a Schmitt trigger operates on the basis of the signal exceeding certain threshold voltage levels. Because of this rising DC amplitude, the phasing of the received signals will be somewhat erroneous compared to the original signal. Another problem is that if the Schmitt trigger threshold levels are set low enough to reconstruct these flattened pulse waveforms, noise and residual transmission signals can also be received by the transceiver, resulting in erroneous information. Also, heretofore Schmitt triggers in such circuits required D.C. coupling.
Yet another problem is that the coupling of the transceiver to the transmission media is required to be of low capacitance, especially in the off (i.e. non-conducting) state. Heretofore this was thought to be possible only by using difficult to find, fragile, and expensive, low capacitance transistors and diodes.